Washington -LRB- CNN -RRB- -- `` I am not a member of any organized political party , '' Will Rogers once joked . `` I am a Democrat . ''

Plenty has been written on the current fight for the soul of the Republican Party . But as President Barack Obama unofficially rings in the sixth year of his administration with Tuesday 's State of the Union address , another intriguing struggle may be emerging -- between moderates and progressives on the Democratic side .

It 's not a divide on a par with the cavernous pre-New Deal divisions of Rogers ' day . Not even close . But it is taking on growing importance as Democrats , frustrated with unyielding Washington gridlock on virtually every major issue , start to look past the current administration .

More specifically , as Democrats look toward the future , there 's sharp disagreement over whether to stick with a Bill Clinton-style centrist agenda or embrace the unabashed left-wing populism most notably personified by Sen. Elizabeth Warren of Massachusetts and New York Mayor Bill de Blasio .

`` We are seeing a split in the Democratic Party between self-described progressives and the old guard , '' Brown University political scientist Wendy Schiller told CNN . `` If you dig deep , you see that these are old dividing lines based on economics , region , and culture . ''

Policy ramifications aside , more liberal Democrats worry that a return to Clinton-era centrism will deflate the party 's base . More moderate Democrats fear a full embrace of Warren - and de Blasio-style liberalism will herald a return to the party 's McGovern-Mondale-Dukakis era political wilderness .

Whichever side wins this fight , one thing is already clear : Six years after flocking to the banner of Obama 's `` hope and change '' candidacy , a lot of progressives are disappointed in the current Democratic administration .

`` It 's a mixed legacy , '' said Adam Green , head of the Progressive Change Campaign Committee , a Washington-based liberal group . `` In some areas , like ending two wars , -LRB- Obama 's -RRB- presidency has represented a return to some sanity . But on the core issue of corporate power and a government that fights for the little guy , this administration so far has had a lot of missed opportunities . ''

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Among other things , Green criticized Obama for going along with `` watered down Wall Street reform '' and `` a Republican version of health care reform . ''

Susan Schorin , a 68-year-old political activist from Virginia , told CNN she is particularly disappointed with Obama 's willingness to consider changes to Social Security that could lead to a reduction in future benefits .

`` I feel betrayed , '' said Schorin , a member of Green 's organization and a volunteer for the Obama campaign group Organizing For America . Obama `` was so enthusiastic and identified with saving these programs for people who really need them . He was extremely verbal about it . ''

Obama 's a `` great man , '' Schorin insisted . But he `` turned his back on me and on all of the people that voted for him on that promise . ''

On the other side , the president and vice president of the centrist think tank Third Way published an op-ed in The Wall Street Journal in December warning that de Blasio - and Warren-style liberalism would be `` disastrous for Democrats . ''

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`` While New Yorkers think of their city as the center of the universe , the last time its mayor won a race for governor or senator -- let alone president -- was 1869 , '' they wrote .

And `` what works in midnight-blue Massachusetts ... has n't sold on a national level since -LRB- JFK 's election in -RRB- 1960 . ''

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In another dig at Warren , they argued that plans to boost Social Security benefits are `` exhibit A of this populist political and economic fantasy . ''

One week later , Warren fired off a letter to the heads of six major banks , pushing them to disclose any contributions the banks may have made to think tanks .

`` Shareholders have a right to know how corporate resources are spent , and , even more importantly , policymakers and the public should be aware of your contributions and evaluate the work of the think tanks accordingly , '' she wrote .

Meanwhile , with Obama 's last campaign now well behind him , several left-leaning groups that had previously kept quiet for the sake of the President 's re-election are now echoing Green 's criticism and bucking the administration on a number of issues .

Earlier this month , a coalition of 18 environmental organizations sent the White House a letter warning that Obama 's `` all of the above '' energy strategy -- one that embraces so-called `` green energy '' and more domestic extraction of traditional fossil fuels -- is unacceptable for those sounding the alarm about climate change .

`` An ` all of the above ' strategy is a compromise that future generations ca n't afford , '' the letter declared . `` It increases environmental injustice while it locks in the extraction of fossil fuels that will inevitably lead to a catastrophic climate future . ''

Liberal groups made their displeasure clear last year when Obama considered Larry Summers -- a key member of Bill Clinton 's economic team -- to head the Federal Reserve . Obama ultimately went with Janet Yellen .

A number of progressive leaders also pushed back against Obama 's proposed airstrikes in Syria , and made clear their unhappiness with the recent budget deal that froze spending on key liberal domestic priorities .

Obama , who originally emerged from the more liberal wing of the Democratic Party , appears sympathetic at least to the economic populism arguments . In a speech in December to the left-leaning Center for American Progress , the President made clear that America 's growing wealth disparity would be a focus of his remaining time in office .

`` Increasing inequality is most pronounced in our country and it challenges the very essence of who we are as a people , '' he declared .

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While divided Democrats will soon begin their search for a new standard-bearer , political observers warn not to make too much of the differences now bubbling to the surface .

`` Both parties have a far greater degree of programmatic consensus than they did 40 or 50 years ago . This is one reason why the parties have polarized and why it is often difficult to achieve bipartisan compromises , '' noted George Washington University political scientist John Sides .

`` The differences between Obama , Hillary Clinton , Warren -LRB- and others -RRB- are mostly at the margins , '' he argued . `` There is far more agreement than disagreement on the fundamental issues . ''

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`` By their nature , political parties are coalitions of mostly like-minded voters and groups , '' Johns Hopkins University political scientist Adam Sheingate added .

`` It 's not uncommon for there to be differences on policy issues within the coalition , or a range of opinions on issues that span part of the political spectrum . ''

Regardless , Schiller argued that the financial demands of modern presidential campaigns may ultimately spell defeat for de Blasio - and Warren-style economic populism .

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`` There is a strong voice within the Democratic Party that believes Barack Obama has not done enough to protect the average working voter , '' she said .

`` For now Elizabeth Warren personifies that voice , but when the pressure to raise really big money for 2016 rears its ugly head -LRB- and -RRB- the Democrats need to be competitive for Wall Street money , her voice may very well get drowned out by the pragmatic wing of the party . ''

Schiller also noted that around the dawn of the last century , the Republican Party wrestled with a similar split on economic issues . The outcome of that struggle eventually helped fuel decades of dominance for Franklin Roosevelt 's New Deal coalition .

`` History tells us that danger lurks for both the Democrats and the Republicans if they fail to heal their internal divisions , '' she warned .

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Dems disagree over whether to stick with centrist agenda or embrace left-wing populism

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Obama `` so far has had a lot of missed opportunities , '' progressive leader says

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When 2016 rolls around , pragmatists might drown out progessives , expert says